The science of detecting infrared and other optical signals by electrical processes is over one hundred fifty years old. It has progressed from the use of radiation thermocouples to thermopiles, to photoconductors (originally of selenium and subsequently of numerous other materials), and photovoltaics. A brief review of the history of infrared detection, in particular, may be found in H. Levinstein et al, "Infrared Detectors in Remote Sensing," Proceedings of the I.E.E.E., Vol. 63, No. 1, January 1965, at 6.
Infrared detectors are used in a wide variety of applications. They are used, for example, to assist energy conservation efforts by sensing heat loss from buildings; to facilitate preventive maintenance of mechanical equipment by detecting localized overheating (e.g., of bearings) before equipment failure occurs; to facilitate diagnosis of certain injuries and diseases by detecting abnormal tissue temperatures; to detect signals transmitted over optical communications channels such as optical fibers; to detect signals of physical and astrophysical interest; and to locate heat sources of various types under a wide variety of conditions, including conditions of poor visibility.
In many of these applications, such detectors must satisfy stringent requirements; these include goals of small size, low power consumption, high sensitivity to incident radiation at the wavelength(s) of interest, fast response and low noise. Of these, high sensitivity and low noise frequently are the dominant requirements. To achieve low noise and high sensitivity, detectors are made as small as possible and are often cooled to within a few tens of degrees Kelvin of absolute zero. The cooling equipment, of course, adds sustantially to space and power consumption; and the cooler the detector must be, the greater the overhead imposed by the cooling equipment.
With detectors in which optical absorption takes place in a semiconductor, sensitivity and performance generally increase with decreasing thickness of the active semiconductor material, provided there is strong absorption of the incident radiation. In the prior art, however, strong absorption has not been attained unless the thickness of the detector is greater than the absorption length of the incident radiation in the semiconductor material; and absorption length is a fundamental physical property of that material, so thickness constraints have been a significant obstacle to improved performance.
The past four decades have seen much progress in the development of high performance infrared detectors and optical detectors for other wavelengths. A great deal of this progress is attributable to improvements in techniques for fabricating the photoconductive and photovoltaic semiconductor materials which are used in such detector elements. Less progress has been made in the area of detector structure. Thus the photodiode family includes just a few types of structures (albeit with variations within each type); these include p-n junction diodes, p-ipn diodes, metal-semiconductor diodes (e.g., Schottky barrier), and heterojunction diodes. In some of these photodiodes, light absorption takes place in a semiconductor material, while in others the absorption occurs in a thin metal film layer; each has its advantages and disadvantages, including wavelengths of superiority, The present invention relates most immediately to the type of photodetector which uses a semiconductor material for absorption of light, but it is also useful with some of the photodetector structures in which light is absorbed in a metal film.
It is therefore an object of the invention to provide an improved photodetector structure.
Another object of the invention is to provide a photodetector structure using an active layer of optically absorptive material whose thickness is substantially less than the absorption length of the active material.
Another object of the invention is to provide a photodetector structure exhibiting increased sensitivity to incident optical radiation.
A further object of the invention is to provide a photodetector structure which allows the detector to be operated at higher temperatures than has heretofore been practical with comparable materials.
Another object of the invention is to provide a photodetector structure exhibiting low noise.
A still further object of the invention is to provide a photodetector structure which dissipates less power than prior art photoconductors using comparable materials.
Yet another object of the invention is to provide a photodetector structure employing a thin layer of a superlattice material as the active, optically absorptive material.
A further object of the invention is to provide a photodetector structure providing various combinations of the foregoing features and characteristics.